Lithographic apparatus and device manufacturing method

ABSTRACT

A liquid supply system for an immersion lithographic projection apparatus is disclosed in which a space is defined between the projection system, a barrier member and a substrate. The barrier member is not sealed such that, during use, immersion liquid is allowed to flow out the space and between the barrier member and the substrate.

This application is a continuation of co-pending U.S. patent applicationSer. No. 10/743,271 filed Dec. 23, 2003, the entire contents of which ishereby incorporated by reference.

FIELD

The present invention relates to a lithographic apparatus, a devicemanufacturing method and device manufactured thereby.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a patterning device, such as a mask, may be used togenerate a circuit pattern corresponding to an individual layer of theIC, and this pattern can be imaged onto a target portion (e.g.comprising part of, one or several dies) on a substrate (e.g. a siliconwafer) that has a layer of radiation-sensitive material (resist). Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through the projection beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.)

However, submersing the substrate or substrate and substrate table in abath of liquid (see for example U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidsupply system (the substrate generally has a larger surface area thanthe final element of the projection system). One way which has beenproposed to arrange for this is disclosed in PCT patent application WO99/49504, hereby incorporated in its entirety by reference. Asillustrated in FIGS. 2 and 3, liquid is supplied by at least one inletIN onto the substrate, preferably along the direction of movement of thesubstrate relative to the final element, and is removed by at least oneoutlet OUT after having passed under the projection system. That is, asthe substrate is scanned beneath the element in a −X direction, liquidis supplied at the +X side of the element and taken up at the −X side.FIG. 2 shows the arrangement schematically in which liquid is suppliedvia inlet IN and is taken up on the other side of the element by outletOUT which is connected to a low pressure source. In the illustration ofFIG. 2 the liquid is supplied along the direction of movement of thesubstrate relative to the final element, though this does not need to bethe case. Various orientations and numbers of in- and out-letspositioned around the final element are possible, one example isillustrated in FIG. 3 in which four sets of an inlet with an outlet oneither side are provided in a regular pattern around the final element.

SUMMARY

In addition to the solution described above, a liquid supply system in asecond solution may be provided that comprises a seal member whichextends along at least a part of a boundary of the space between thefinal element of the projection system and the substrate table. The sealmember is substantially stationary relative to the projection system inthe XY plane though there may be some relative movement in the Zdirection (in the direction of the optical axis). A seal is formedbetween the seal member and the surface of the substrate. In anembodiment, the seal is a contactless seal such as a gas seal. Such asystem is disclosed in U.S. patent applications Ser. No. 10/705,805 andU.S. Ser. No. 10/705,783, both hereby incorporated in their entirety byreference.

A third solution that may be provided comprises a member attached to theprojection system which forms a hollow space underneath the finalelement of the projection system. The bottom of the member is providedclose enough to the surface of the substrate such that capillary forcesare strong enough to contain the immersion liquid in the hollow spacecreated by the member between the final element of the projection systemand the substrate.

While all of the above localized area solutions overcome the problem ofneeding to accelerate a large body of liquid, each of the solutions maybe improved. For example, the first solution has proved to be hard toimplement without large and uncontrolled liquid spillage. A furtherexample is that the second and third solutions may deleteriouslytransmit disturbance forces to the substrate and/or projection systembecause of their close interaction with the surface of the substratewhich is necessary in order to keep the liquid in the space.Furthermore, each of the solutions may not be particularly well suitedfor imaging objects of different height on the substrate table, such asthrough lens sensors. There is not a great deal of available space withthese systems between the projection system and the substrate andbuilding a liquid supply system which can operate at high NA can bedifficult. Each of the solutions may present difficulties with theimaging of edge portions of the substrate, such as the gas seal maybecome unbalanced when it is partly positioned over the edge of thesubstrate and/or the capillary force can be lost when imaging edgeportions. These solutions all work best with a low free working distance(which would advantageously be higher) and with high fluid pressures(which would advantageously be lower).

Accordingly, it would be advantageous, for example, to provide immersionliquid to a space between a projection system and the substrate withoutsome or all of the above problems, in particular avoiding significanttransmission of disturbance forces to the substrate.

According to an aspect, there is provided a lithographic apparatuscomprising:

an illumination system configured to provide a beam of radiation;

a support structure configured to hold a patterning device, thepatterning device configured to impart the beam with a pattern in itscross-section;

a substrate table configured to hold a substrate;

a projection system configured to project the patterned beam onto atarget portion of the substrate; and

a liquid supply system configured to provide an immersion liquid to aspace between the substrate and the projection system, the liquid supplysystem comprising a barrier member extending along at least a part ofthe boundary of the space and being in a position relative to an objecton the substrate table so that any capillary pressure generated by theimmersion liquid between the barrier member and the object is not largeenough to constrain the immersion liquid in the space,

wherein no seal is provided between the barrier member and the object.

In this way, in use, the immersion liquid is allowed to leak out of thespace between the bottom of the barrier member and the substrate and isthereby not constrained in the space. Thus, the transmission ofdisturbance forces between the projection system, the barrier member andthe substrate may be reduced or minimized. Furthermore, a high rate ofliquid replenishment in the space may be possible without the necessityfor the use of high liquid pressures. The force in the direction of theoptical axis on the substrate table may also be reduced and be moreconstant in comparison to other liquid supply systems. Also, unlike withother supply systems, the imaging of edge portions may become easier ascomplicated measures are not necessary as the barrier member passes overthe edge of the substrate as there is no seal to be disturbed at theedge of the substrate. The simplicity of the barrier member may beincreased as only liquid inlets are required and no gas supplies. Thelack of gas supplies means that the chance of bubble formation in theimmersion liquid which can deleteriously affect the imaging quality mayreduced or minimized. Finally, a larger free working distance (thedistance between the projection system and the substrate) may beincreased compared to other supply systems.

In an embodiment, the apparatus further comprises at least one outlet toremove immersion liquid, the outlet being radially outwardly of thebarrier member. In this way immersion liquid which has spilled from thelocalized area of the supply system (i.e. the area under the projectionsystem) may be collected without adding to the complexity of the barriermember and without substantially transferring disturbance forces to thesubstrate and/or substrate table. In one embodiment the outlet is on thesubstrate table.

In an embodiment, the barrier member is mechanically isolated from theprojection system so that disturbance forces are not automaticallytransmitted to the projection system by the barrier member. In anembodiment, the barrier member is connected to a base frame whichsupports the substrate table and/or a projection system frame whichsupports the projection system. In an embodiment, the barrier member isfree to move in the direction of an optical axis of the projectionsystem.

For flexibility of the system, the apparatus may comprise an actuatorconfigured to adjust the height and/or tilt of the barrier memberrelative to the substrate.

According to a further aspect, there is provided a device manufacturingmethod comprising:

providing an immersion liquid to a space between a substrate on asubstrate table and a projection system, a bather member extending alongat least a part of the boundary of the space;

allowing immersion liquid to leak between the barrier member and anobject on the substrate table by positioning at least one of the barriermember and the object so that any capillary pressure generated by theimmersion liquid between the barrier member and the object is not largeenough to constrain the immersion liquid in the space; and

projecting a patterned beam of radiation onto a target portion of thesubstrate using the projection system.

According to a further aspect, there is provided a device manufacturedaccording to the above-referenced device manufacturing method and/or bythe above-referenced lithographic apparatus.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion”, respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm).

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a projection beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the projection beam may not exactly correspond to thedesired pattern in the target portion of the substrate. Generally, thepattern imparted to the projection beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit.

A patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned. In each example of a patterning device, thesupport structure may be a frame or table, for example, which may befixed or movable as required and which may ensure that the patterningdevice is at a desired position, for example with respect to theprojection system. Any use of the terms “reticle” or “mask” herein maybe considered synonymous with the more general term “patterning device”.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “lens” herein may be considered assynonymous with the more general term “projection system”.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation, and such components may also be referred to below,collectively or singularly, as a “lens”.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 illustrates, in cross-section, a liquid supply system of theprior art;

FIG. 3 illustrates, in plan, the liquid supply system of FIG. 2; and

FIG. 4 illustrates the liquid supply system according to an embodimentof the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to aparticular embodiment of the invention. The apparatus comprises:

an illumination system (illuminator) IL for providing a projection beamPB of radiation (e.g. UV radiation);

a first support structure (e.g. a mask table) MT for supporting apatterning device (e.g. a mask) MA and connected to a first positioningdevice PM for accurately positioning the patterning device with respectto item PL;

a substrate table (e.g. a wafer table) WT for holding a substrate (e.g.a resist-coated wafer) W and connected to a second positioning device PWfor accurately positioning the substrate with respect to item PL; and

a projection system (e.g. a refractive projection lens) PL for imaging apattern imparted to the projection beam PB by patterning device MA ontoa target portion C (e.g. comprising one or more dies) of the substrateW.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above).

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising for examplesuitable directing mirrors and/or a beam expander. In other cases thesource may be integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise an adjusting device AM for adjusting theangular intensity distribution of the beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator ILgenerally comprises various other components, such as an integrator INand a condenser CO. The illuminator provides a conditioned beam ofradiation, referred to as the projection beam PB, having a desireduniformity and intensity distribution in its cross-section.

The projection beam PB is incident on the mask MA, which is held on themask table MT. Having traversed the mask MA, the projection beam PBpasses through the lens PL, which focuses the beam onto a target portionC of the substrate W. With the aid of the second positioning device PWand position sensor IF (e.g. an interferometric device), the substratetable WT can be moved accurately, e.g. so as to position differenttarget portions C in the path of the beam PB. Similarly, the firstpositioning device PM and another position sensor (which is notexplicitly depicted in FIG. 1) can be used to accurately position themask MA with respect to the path of the beam PB, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe object tables MT and WT will be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the positioning devices PM and PW.However, in the case of a stepper (as opposed to a scanner) the masktable MT may be connected to a short stroke actuator only, or may befixed. Mask MA and substrate W may be aligned using mask alignment marksMl, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following preferred modes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theprojection beam is projected onto a target portion C in one go (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the projection beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the projection beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizes aprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 4 shows a liquid supply system according to an embodiment of thepresent invention. The liquid supply system comprises a barrier member10. The barrier member 10 surrounds the final element of the projectionsystem PL. The barrier member 10 extends along at least part of theboundary of a space between the final element of the projection systemPL and the substrate W. The space generally bounded by the projectionsystem PL, the barrier member 10 and the substrate W is filled with animmersion liquid 5.

There is no seal between the barrier member 10 and the substrate W. Thebarrier member 10 is far enough above the substrate W such thatcapillary forces do not act to contain the immersion liquid 5 in thespace between the final element of the projection system PL and thesubstrate W bounded by the barrier member 10; in use, such as duringscanning or stepping, immersion liquid 5 flows out under the barriermember 10 as illustrated because any capillary pressure generated in thegap between the bottom of the barrier member 10 and the top of thesubstrate W is not large enough to contain the liquid. The barriermember 10 may be connected to the base frame BF, the projection systemframe RF and/or another frame. In an embodiment, the barrier member 10is mechanically isolated from the projection system PL so thatdisturbances forces imparted on or generated by the barrier member 10can be prevented or at least limited from being transmitted to theprojection system PL.

No provision is made, for example, during scanning, to seal the space 5to avoid loss of immersion liquid. The level of immersion liquid in thepassage between the projection system PL and the barrier member 10 ismaintained as constant as possible by the provision of immersion liquidthrough one or more inlet ports 20.

The barrier member 10 may be comprised solely of the one or more inletports 20 so that the arrangement is similar to that shown in FIG. 3except the outlet ports OUT are inlet ports. Thus, the one or more inletports 20 are positioned circumferentially around the optical axis of theapparatus.

Immersion liquid 5 is provided to the space through the one or moreinlet ports 20 which is/are formed on a bottom inner edge of thegenerally annular shaped barrier member 10. The barrier member 10 may beother shapes whether closed (e.g., rectangular) or open (e.g.,U-shaped). A chamber 24 provided between the one or more inlet ports 20and the supply of immersion liquid 22, ensures that immersion liquid isprovided into the space at an even pressure around the inner peripheryof the barrier member 10 even though the sources of immersion liquid maybe provided via one or more discrete channels 22 (as opposed to, forexample, a continuous groove). The one or more inlet ports 20 may be acontinuous groove.

As is illustrated in FIG. 4, the barrier member 10 extends below the oneor more inlet ports 20 radially outwardly of the one or more inlet ports20, whereas radially inwardly the barrier member 10 is further displacedfrom the substrate W than on the other side of the one or more inletports 20. This design reduces the chance of gas inclusion at the one ormore inlet ports 20.

In order to avoid significant capillary forces being generated byimmersion liquid 5 filling the gap between the bottom of the barriermember 10 and the top of the substrate W, the bottom of the barriermember is, in an embodiment, at least 50 μm from the substrate W. In anembodiment, the distance 30 is substantially 100 μm or substantially 150μm. A distance 30 of over 300 μm (even 400 μm) may not be uncommon, forinstance during scanning of sensors 70. These are typical distances ifthe immersion liquid is water. The required distance may be differentfor other liquids. The barrier member 10 may be moveable in the Z axis40 such that the distance 30 between the substrate W (or any otherobject) and the barrier member 10 can be adjusted. The barrier membermay also be moveable about one or more axes substantially perpendicularto the Z axis 40 such that the tilt between the substrate W (or anyother object) and the barrier member 10 can be adjusted. Alternativelyor in addition, the substrate table WT may be moveable in the Z axis 40to adjust the distance 30 between the substrate W (or any other object)and the barrier member 10 and/or moveable about one or more axessubstantially perpendicular to the Z axis 40 to adjust a tilt betweenthe substrate W (or any other object) and the barrier member 10.

Therefore, in use, the immersion liquid is spilt radially outwardly ofthe barrier member 10 and flows on the substrate (or object (e.g.substrate table WT)) and immersion liquid can be provided through theone or more inlet ports 20 at a low pressure. If the immersion liquid iswater, a pressure of about 1000 Pa in the one or more inlet ports 20 isabout right and with a suitable restriction between the supply channeland the containment under the projection system, a pressure of about 100Pa can be achieved under the projection system. Thus, a small constant Zforce on the substrate table WT is provided of about 50 mN.

In order to remove the immersion liquid which has been spilled from theliquid supply system, one or more of the outlets 60, 63, 66 may beprovided. The outlets are positioned radially outwardly of the barriermember 10 and do not form part of the barrier member 10. Any arrangementcan be used for outlets and three possibilities are shown in FIG. 4. Atypical outlet 60 might be one which is connected either to the baseframe BF or the projection system frame RF (shown in FIG. 1) and whichremoves liquid from the surface of the substrate W or substrate table WTor a substrate table mounted sensor 70 or a shutter member 80 (describedin more detail hereafter). Alternatively or in addition, one or moreoutlets 63 may be provided in the top surface of the substrate table WTand/or one or more outlets 66 can be provided at the edge of thesubstrate W. In order to avoid spillage, a rim 50 may be provided aroundthe substrate table WT.

The liquid supply system may be used for imaging of through lens sensors70 mounted on the substrate table WT as well as with a shutter member80, which can be attached to the bottom of the barrier member 10 througha vacuum source, by magnets, etc. which ensures that the final elementof the projection system PL is maintained wet during substrate W swap.Shutter members are described in more detail in U.S. patent applicationSer. No. 10/705,785, herein incorporated in its entirety by reference.For both the sensors 70 (which may be, for example, transmission imagesensors (TIS)) and the shutter member 80, the barrier member 10 may belowered towards the substrate W (and/or the substrate table WT may beraised towards the barrier member 10). No scanning takes place when thesensors 70 are imaged and during this time (and other times when noscanning takes place) the pressure of liquid in the barrier member 10can be reduced such that a meniscus can be formed between the barriermember 10 and the substrate table WT so that less liquid escapes as wellas perhaps lowering the barrier member 10.

Adaptive height control of the barrier member 10 could be used (seeEuropean patent application EP 03256643.2, herein incorporated in itsentirety by reference) possibly based on the position of the substratetable WT with respect to the projection system.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1.-37. (canceled)
 38. A lithographic apparatus comprising: a substratetable configured to hold a substrate; a projection system configured toproject a patterned beam onto a radiation-sensitive target portion ofthe substrate; and a liquid supply system configured to provide a liquidto a space between the substrate and the projection system, the liquidsupply system comprising a liquid inlet provided on a boundary of thespace, not provided on the substrate table, and mechanically isolatedfrom the projection system, wherein, in use, the liquid is notsubstantially confined in the space so that liquid can flow out of thespace.
 39. An apparatus according to claim 38, further comprising aliquid outlet radially outwardly of the liquid inlet and outwardly of aperiphery of the substrate when held by the substrate table.
 40. Anapparatus according to claim 39, wherein the liquid outlet is on thesubstrate table.
 41. An apparatus according to claim 38, wherein theliquid inlet is moveable in the direction of an optical axis of theprojection system.
 42. An apparatus according to claim 38, furthercomprising an actuator configured to adjust a height, or a tilt, orboth, of the liquid inlet relative to the substrate.
 43. A lithographicapparatus comprising: a substrate table configured to hold a substrate;a projection system configured to project a patterned beam onto aradiation-sensitive target portion of the substrate; an inlet to providea liquid to a space between the substrate and the projection system, theliquid inlet not provided on the substrate table; and an outlet toreceive liquid, the liquid outlet located radially outwardly, relativeto an optical axis of the projection system during projection of thepatterned beam, of the liquid inlet and of a periphery of the substratewhen the substrate is held by the substrate table, wherein, duringprojection of the patterned beam, liquid flows outwardly from the liquidinlet into the liquid outlet.
 44. An apparatus according to claim 43,wherein the liquid outlet is on the substrate table.
 45. An apparatusaccording to claim 44, wherein the liquid outlet is in a surface of thesubstrate table substantially parallel to a surface of the substrateexposed to the projected patterned beam.
 46. An apparatus according toclaim 43, wherein the liquid outlet is separated from the periphery ofthe substrate by a portion of the substrate table substantiallyco-planar with a surface of the substrate exposed to the projectedpatterned beam.
 47. An apparatus according to claim 43, wherein theliquid inlet is mechanically isolated from the projection system.
 48. Anapparatus according to claim 47, wherein the liquid inlet is moveable inthe direction of an optical axis of the projection system.
 49. Anapparatus according to claim 43, wherein, during projection of thepatterned beam, liquid flows outwardly from the liquid inlet over theperiphery of the substrate at opposing portions of the substrate.
 50. Anapparatus according to claim 43, wherein the liquid inlet is locatedover the substrate during projection of the patterned beam.
 51. Anapparatus according to claim 43, wherein the liquid outlet issubstantially co-planar with a surface of the substrate exposed to theprojected patterned beam.
 52. A lithographic apparatus comprising: asupport structure configured to hold a patterning device, the patterningdevice configured to impart a beam of radiation with a pattern in itscross-section; a substrate table configured to hold a substrate; aprojection system configured to project the patterned beam onto a targetportion of the substrate; and an outlet to receive liquid, the liquidoutlet located outside a periphery of the substrate during projection ofthe patterned beam and having its outlet surface at or below a surfaceof the substrate exposed to the projected patterned beam, wherein,during projection of the patterned beam, the liquid outlet is the firstoutlet into which liquid flows off the substrate.
 53. An apparatusaccording to claim 52, wherein the liquid outlet is on the substratetable.
 54. An apparatus according to claim 53, wherein the liquid outletis in a surface of the substrate table substantially parallel to thesurface of the substrate.
 55. An apparatus according to claim 53,wherein the liquid outlet is substantially co-planar with the surface ofthe substrate.
 56. An apparatus according to claim 52, wherein, duringprojection of the patterned beam, liquid flows outwardly over theperiphery of the substrate at opposing portions of the substrate.
 57. Alithographic apparatus comprising: a support structure configured tohold a patterning device, the patterning device configured to impart abeam of radiation with a pattern in its cross-section; a substrate tableconfigured to hold a substrate; a projection system configured toproject the patterned beam onto a target portion of the substrate; and aliquid system configured to provide, during projection of the patternedbeam, liquid flows outwardly from an interior of the substrate over theperiphery of the substrate at opposing portions of the substrate, theliquid system having an outlet to receive liquid located outside aperiphery of the substrate during projection of the patterned beam.